The invention relates generally to a blow molding process and more particularly a process for the production of a hollow body of thermoplastic material by blow molding.
A process for producing a hollow body from thermoplastic material by blow molding is disclosed in German patent specification No. 2 401 948, wherein a tubular perform is expanded within the closed blowing mold, so as to cause it to bear inside the inside configuration of the mold, by the introduction of a gas mixture which contains an inert gas. The inside surface of the resulting hollow body within the blowing mold is treated to render the wall of the hollow body impervious to fluid, and that operation is effected by means of a reaction gas which is also introduced into the hollow body in the closed mold, as a contituent of the gas mixture containing the inert gas. The reaction gas contained in the gas mixture used is fluorine. Thus, the gas mixture in that process performs two functions: on the one hand it provides for putting the preform into the required shape of the molded article, such as a container or the like, while at the same time it provides for treating the inside wall surface thereof. The first function requires operation to be carried out under comparatively high gas pressure; in many cases, particularly when producing hollow bodies which are of a thick-wall nature and which are also of a complicated configuration, the gas pressure used may be 8 to 10 bars and even higher. However, as the reaction gas which is to be used for producing the desired reactions at the inside wall surface of the hollow body are all such as to give rise to severe environmental pollution, the fact that the process is carried out with elevated pressures means that it is necessary to take particular precautions in order to avoid the danger of environmental pollution, for example in the event of a defective blowing operation which cannot always be avoided when hollow bodies are being produced by means blow molding. It should be appreciated that the danger of environmental pollution arises to an even greater degree by virtue of the fact that, to achieve economical production, the gas mixture which performs the two functions indicated above must be continuously exchanged, within the hollow body, in order in that way also to cool the hollow body in its interior. Furthermore, the fact that operation is carried out under elevated pressure means that a large amount of reaction gas is required for properly performing the process.
In another blow molding process, as disclosed in German laid-open application (DE-OS) No. 29 24 797, the process comprises using a hollow perform of thermoplastic material, the reaction gas being introduced into the preform before the preform is put into the blowing mold, with the aim of causing the desired reaction to take place at the inside wall surface of the preform before the operation of expanding the preform, which is then carried out by introducing into the hollow body, within the blowing mold, a special compressed gas which does not contain any reaction gas. Besides affording the possibility of using the gas mixture containing the reaction gas for also pre-expanding the preform outside the blowing mold, which is conventional practice or even necessary in many situations, the above-indicated process also makes it possible to use a lower pressure for treating the preform and for the pre-expansion operation which may be required. The volume which is to be filled by the reaction gas mixture is lower, even in the pre-expansion operation, than in the first known process discussed above. However, it is not possible to exclude the occurrence of other disadvantages, for example the inner layer of the wall of the hollow body, upon which the reaction gas acts, is subjected to a stretching effect, in the operation of definitively shaping the molded article within the blowing mold, such stretching effect corresponding to that which the preform experiences overall in the final and definitive expansion operation. Such stretching of the inner layer of the wall of the hollow body occurs even when the preform has been subjected to pre-expansion, in the above-indicated manner. It is difficult if not impossible to cause the stretching effect to be uniform over the entire inside wall surface of the hollow body. As such stretching necessarily also causes a reduction in the thickness of the wall of the hollow body, the layer of the wall of the hollow body which is acted upon by the reaction gas also experiences a corresponding variation in its thickness, the extent thereof depending on the degree of stretching that occurs. Because, as already stated, the stretching effect is not regular, the thickness of the inner layer of the wall of the hollow body is ultimately also irregular, in the finished article. It will be appreciated that, where the molded article is for example a container for gas or liquid, the wall of the container must be at least substantially impermeable thereto, and should preferably be completely fluid-tight. However, the impermeability of the molded article will depend on the thickness of the inner layer of the wall of the molded article which has been subjected to the treatment by the reaction gas in order to render it impervious to fluid, so that, in order to achieve the desired substantially and preferably completely impermeable wall for the molded article, the thickness of the inner layer which undergoes treatment by the reaction gas must be so selected that, even in a region in which the wall of the hollow body has been stretched to its greatest degree, the layer in question is still of adequate thickness to be impervious to fluid, on the finished product. That can have the result that in other regions of the wall of the article, the thickness of the layer is greater than that which is necessarily to be provided in order to achieve the desired effect. That means that the operation of treating the wall of the preform by the reaction gas becomes more expensive both in terms of time and cost, either due to the reaction gas requiring a longer period to act on the inner layer of the hollow body or due to the concentration of the reaction gas having to be higher, in comparsion with a process which involves treating the inside wall surface of the finished hollow body with the reaction gas. Added to that is the fact that the preform must be closed off within the blowing mold at at least one end but in many cases also in other regions thereof, by forming a welded seam at the appropriate location. The welding operation to close the preform is generally carried out at the same time as the operation of pinching off excess material from the preform. For that purpose, the components which form the blowing mold having pinching-off edge portions between which on the one hand excess material is pinched off and at the same time the above-mentioned welded seam is produced. It will be appreciated that the quality of the welding achieved is a factor of crucial significance in regard to the suitability of the hollow body for use as a blowing mold preform. The possibility cannot be excluded that, due to the inside wall surface of the preform being chemically affected by the reaction gas, the properties of the inside wall surface of the preform, which are significant in relation to the operation of producing the welded seam, may also be adversely affected. That may also be the case when the preform is subjected to a preexpansion step as in that case the operation of pinching off excess material and the operation of forming the welded seams are carried out only when the blowing mold is closed around the pre-expanded preform.